Method and apparatus for multiplexed video data reading in video library system

ABSTRACT

A video library system capable of shortening the worst response time with respect to each video data reading request, without lowering the possible level of multiplexing. The multiplexed video data reading is achieved by distributedly storing video data divided into segments among a plurality of data storage devices, where each segment is capable of being read out in one time-slot; selectively connecting each data storage devices with one of data transfer targets by a plurality of switches provided in correspondence to the data storage devices; generating a plurality of control time-slot sequences formed by periodic time-slots in correspondence to the data storage devices with mutually displaced phases in each period of the control time-slot sequences; and controlling the data storage devices and switches such that, in response to a request for reading video data from a data transfer target, an earliest available idle time-slot is selected from the control time-slot sequences, and the segments of the requested video data are sequentially read out from the data storage devices at time-slots corresponding to the selected earliest idle time-slot in the control time-slot sequences and transmitted to the requesting data transfer target as the data storage devices are sequentially switched to the requesting data transfer target by the switches.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a video library system for reading aplurality of video data simultaneously from data storage devices such asdisk devices in which many video data including audio data are recordedby being compression coded, which is utilized in the video-on-demandsystem for providing a plurality of video data simultaneously through anetwork in response to requests from a plurality of user terminals.

2. Description of the Background Art

As a conventional multiplexed video data reading device for reading aplurality of video data simultaneously, there has been a proposition ofa video library system as disclosed in Japanese Patent Application LaidOpen No. 4-269087 (1992). FIG. 1 shows a configuration of such aconventional video library system, in which a disk device DK101 isconnected with a plurality of buffer memories BM101, BM102, . . . ,BM132 through a switch SW101, where all of these elements are operatedunder a control of a control device CN101. Here, the buffer memoriesBM101, BM102, . . . , BM132 are provided within terminals TE101, TE102,. . . , TE132 which also include decoders DE101, DE102, . . . , DE132,and monitors TV101, TV102, . . . , TV132, respectively.

In this conventional video library system of FIG. 1, the multiplexedreading operation is controlled according to the timing chart of FIG. 2as follows.

The control device CN101 generates a control time-slot sequence CTS101formed by periodic 32 time-slots TS101, TS102, . . . , TS132constituting one period, and within each period, different video dataare read out from the disk device DK101 at different time-slots in sucha manner that the video data for the buffer memory BM101 of the firstterminal TE101 is read out at the time-slot TS101, the video data forthe buffer memory BM102 of the second terminal TE102 is read out at thetime-slot TS102, and so on. Accordingly, the transfer target of thevideo data read out from the disk device DK101 is sequentially switchedby the switch SW101 among the buffer memories BM101, BM102, . . . ,BM132. The video data received at the buffer memories BM101, BM102, . .. , BM132 are then decoded into video signals by the respective decodersDE101, DE102, . . . , DE132 and displayed by the respective monitorsTV101, TV102, . . . , TV132, so as to realize the multiplexed videodisplay at a plurality of terminals TE101, TE102, . . . , TE132.

As shown in FIG. 2, in this conventional video library system, inresponse to the video data reading request from each terminal, an idletime-slot not occupied by the other terminals is searched out in eachperiod of the control time-slot sequence, and thereafter the reading ofthe requested video data is carried out periodically by using thesearched out time-slot in each period of the control time-slot sequence.Consequently, there has been a waiting period for the video data readingoperation between a time at which the video data reading request hasarrived and a time at which the idle time-slot is found in the controltime-slot sequence. Here, the response time required from a time atwhich the video data reading request is made to a time at which thereading of the requested video data from the disk device DK101 isactually completed for the first time-slot part becomes worst when thereis only one idle time-slot in one period of the control time-slotsequence and the video data reading request arrives immediately afterthat idle time-slot, in which case the waiting period is as long as theduration of one period in the control time-slot sequence.

For example, in FIG. 2, consider a case in which the n-th terminalissued the video data reading request REQ101 at a timing of thecompletion of the time-slot TS132. First, the idle time-slot is searchedamong the following time-slots for one period starting from that timing.Here, suppose that the time-slots TS101 to TS131 are already occupiedand only the time-slot TS132 is idle. Then, The video data readingrequest REQ101 is going to be allocated to this time-slot TS132, so thatthe execution of the video data reading operation must wait until thistime-slot TS132 in the next period. In this case, the monitor displaystart timing RES101 at the n-th terminal is delayed from the timing ofthe video data reading request REQ101 by exactly one period part asindicated in FIG. 2 as TIME101. When a number of time-slots in oneperiod is 32 and a duration of each time-slot is 125 ms, the duration ofthis delay time TIME101 amounts to 4 sec.

In such a conventional multiplexed video data reading scheme, in orderto shorten the worst response time, it has been necessary to shorten theduration of one period by shortening the duration of each time-slot.

On the other hand, at the boundary of the adjacent time-slots in thetime division multiplexed data reading control scheme, in general, thereis an overhead time required for switching the data to be read out atthe data storage device side. For example, in a case of using themagnetic disk device as the data storage device, there is a need toswitch the reading target block on the disk, and there is an overheadtime due to the head seek and the sector search for this purpose.Consequently, when the duration of each time-slot is shortened, a rateof the actual data reading time within each time-slot is going to berelatively lowered, and this in turn reduces the effective average datareading speed at the data storage device.

For example, when the overhead time is constantly 50 ms, for thetime-slot with a duration of 125 ms, the reading efficiency at the diskdevice is going to be 60% (=(125-50)/125), but when the duration of thetime-slot is shortened to 62.5 ms, the reading efficiency at the diskdevice is going to be lowered to 20% (=(62.5-50)/62.5), so that theaverage data reading speed is going to be reduced to 1/3.

Thus, in the conventional multiplexed video data reading scheme, therehas been a problem that, when each time-slot is shortened in order toshorten the worst response time, the average data reading speed at thedata storage device is lowered in turn, and as a result the possiblelevel of multiplexing is going to be lowered.

In addition, in the conventional video library system, each terminal isgiven no information concerning the video data contents, so that inorder to distinguish the video data having similar attributes such asthe video data having similar titles, it has been necessary for eachterminal to carry out an operation to actually make an access to acenter to read out these video data and confirm their contents.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a videolibrary system capable of shortening the worst response time withrespect to each video data reading request, without lowering thepossible level of multiplexing.

It is another object of the present invention to provide a video librarysystem capable of displaying a complete video data at a terminal evenwhen the video data provided from a center has some skipping videosegments from a top section of the video data.

It is another object of the present invention to provide a video librarysystem capable of confirming each video data at a terminal side withoutrequiring an access to a center to read out the entire video data.

According to one aspect of the present invention there is provided anapparatus for multiplexed video data reading, comprising: N data storagemeans for distributedly storing video data divided into segments, whereeach segment is capable of being read out in one time-slot and N is aninteger greater than one; N switch means, provided in correspondence tosaid N data storage means, for selectively connecting each data storagemeans with one of a plurality of data transfer targets; and a controldevice for generating N control time-slot sequences formed by periodictime-slots in correspondence to said N data storage means with mutuallydisplaced phases in each period of said N control time-slot sequences,and controlling said N data storage means and said N switch means suchthat, in response to a request for reading one video data from one datatransfer target, an earliest available idle time-slot is selected fromsaid N control time-slot sequences, and the segments of said one videodata are sequentially read out from said N data storage means attime-slots corresponding to said earliest idle time-slot in said Ncontrol time-slot sequences and transmitted to said one data transfertarget as said N data storage means are sequentially switched to saidone data transfer target by said N switch means.

According to another aspect of the present invention there is provided amethod of multiplexed video data reading, comprising the steps of:distributedly storing video data divided into segments among N datastorage means, where each segment is capable of being read out in onetime-slot and N is an integer greater than one; selectively connectingeach data storage means with one of a plurality of data transfer targetsby N switch means provided in correspondence to said N data storagemeans; generating N control time-slot sequences formed by periodictime-slots in correspondence to said N data storage means with mutuallydisplaced phases in each period of said N control time-slot sequences;and controlling said N data storage means and said N switch means suchthat, in response to a request for reading one video data from one datatransfer target, an earliest available idle time-slot is selected fromsaid N control time-slot sequences, and the segments of said one videodata are sequentially read out from said N data storage means attime-slots corresponding to said earliest idle time-slot in said Ncontrol time-slot sequences and transmitted to said one data transfertarget as said N data storage means are sequentially switched to saidone data transfer target by said N switch means.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a conventional apparatus formultiplexed video data reading in a video library system.

FIG. 2 is a timing chart for an exemplary multiplexed video data readingoperation in the conventional apparatus of FIG. 1.

FIG. 3 is a schematic block diagram of an overall configuration of avideo library system according to the present invention.

FIG. 4 is a schematic block diagram of the first embodiment of anapparatus for multiplexed video data reading in a video library systemaccording to the present invention.

FIG. 5 is a diagram for explaining a manner of storing video segments ofeach video data into a plurality of disk devices in the apparatus ofFIG. 4

FIG. 6 is a timing chart for an exemplary multiplexed video data readingoperation in the apparatus of FIG. 4.

FIG. 7 is a schematic block diagram of the second embodiment of anapparatus for multiplexed video data reading in a video library systemaccording to the present invention.

FIG. 8 is a schematic block diagram of the third embodiment of anapparatus for multiplexed video data reading in a video library systemaccording to the present invention.

FIG. 9 is a flow chart for the operation of a time-slot selection unitin the apparatus of FIG. 8.

FIG. 10 is a timing chart for an exemplary multiplexed video datareading operation in the apparatus of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the embodiments of the video library system according to thepresent invention will be described in detail.

First, the video library system of the present invention has an overallconfiguration as shown in FIG. 3, which generally comprises a center 1and a plurality of terminals 11, 11', etc. Here, the center 1 includes aplurality of data storage devices 3 for storing video data 2, aswitching unit 4 for switching connections between the data storagedevices 3 and the terminals 11, 11', etc., and a control device 5equipped with a coding scheme distinguishing unit 6 and a time-slotselection unit 7. On the other hand, the terminal 11 includes a buffermemory 21 for receiving the video data transmitted from the center 1, adata memory unit 31 for storing a partial video data for each video data2, a data composition unit 41 for composing the complete video data fromthe received video data in the buffer 21 and the partial video data inthe data memory unit 31, and a data selection unit 51 for selectivelyoutputting either the complete video data obtained by the datacomposition unit 41 or the partial video data stored in the data memoryunit 31. Each of the other terminals 11', etc. also includes the similarelements.

In this video library system of FIG. 3, a plurality of data storagedevices 3 are provided while each video data 2 is divided into segmentssuch that each segment can be read out in one time-slot at each datastorage device 3, and the divided segments of each video data 2 arestored by being distributed among the plurality of data storage devices3. On the other hand, the switching unit 4 includes a plurality ofswitches SW1, SW2, etc. in correspondence to the plurality of datastorage devices 3 such that each data storage device 3 can be connectedwith any desired terminal 11.

Then, in order to control the reading of the distributedly stored videodata, a plurality of time-slot sequences having an identical period aregenerated by the control unit 5 in correspondence to the plurality ofthe data storage device 3, where these time-slot sequences have mutuallydisplaced phases within each period. In response to the video datareading request from each terminal, the earliest unused idle time-slotis selected from all the time-slot sequences, and the video data readingoperation for the requested video data is carried out under the controlof the control unit 5, starting from the data storage device 3corresponding to the time-slot sequence containing the selectedtime-slot, by using the selected time-slot in each time-slot sequence,while appropriately controlling the switches of the switching unit 4 bythe control unit 5 to transmits the video data sequentially read outfrom the plurality of data storage devices 3 to the requesting terminal.

Thus, in this video library system of FIG. 3, the time divisionmultiplexed reading of a plurality of video data is realized byoperating a plurality of sets of the data storage devices and theswitches to read out the video segments in parallel, and composing thevideo data from the read out video segments at the buffer memory of eachterminal. Here, the video data reading operation for any video data canbe started by using any available set of the data storage device and theswitch at each period, provided that the skipping of some video segmentsat a top section of the video data is tolerated. In this manner, it ispossible to search the idle time-slot from all of the plurality oftime-slot sequences in mutually displaced phases, in parallel. In thiscase, the idle time-slot can be found within each phase of each period,so that the worst response time can be shortened to be equal to theduration of each phase of the period of the time-slot sequences.

As for the possible skipping of some video segments at a tope section ofthe video data, which occurs in a case the video data reading operationis started from the video segment other than the top video segment, foras many as (a number N of data storage devices-1) pieces of videosegments at most, each terminal 11 is provided with the data memory unit31 for storing the partial video data covering at least the maximumpossibly skipped video segments of each video data, which can beutilized for compensating the actually skipped video segments in thevideo data received from the center in composing the complete video dataat the data composition unit 41.

This partial video data of each video data stored in the data memoryunit 31 of each terminal 11 can also be utilized in confirming eachvideo data at the terminal side, so that each terminal 11 also has adata selection unit 51 capable of selectively outputting the partialvideo data stored in the data memory unit 31.

On the other hand, the skipping of the video segments can cause aserious problem in a case of using the coding scheme in which the otherframe data referencing range at a time of each frame decoding isvariable and not necessarily closed within one segment, such as the H.261 coding, because the skipped video segments can affect the decodingof the other not skipped video segments as well.

For this reason, the control unit 5 is equipped with the coding schemedistinguishing unit 6 which distinguishes the coding scheme in which theother frame data referencing range at a time of each frame decoding isalways closed within one segment and the coding scheme in which theother frame data referencing range at a time of each frame decoding isnot necessarily closed within one segment, and the time-slot selectionunit 7 for selectively switching the idle time-slot selection rulesaccording to the coding schemes. Here, the coding scheme distinguishingunit 6 can easily distinguish the coding scheme by reading out theinformation in the header portion of the communication data for example.

As a result, the idle time-slot is selected from any of the plurality oftime-slot sequences in a case of the coding scheme in which the otherframe data referencing range at a time of each frame decoding is alwaysclosed within one segment, so as to realize the shortening of theresponse time, while the idle time-slot is selected only from thetime-slot sequence corresponding to the data storage device storing thefirst segment in a case of the coding scheme in which the other framedata referencing range at a time of each frame decoding is notnecessarily closed within one segment, so as to prevent the skipping ofthe video segments from the top section of each video data.

Now, the embodiments illustrating various aspects of this video librarysystem of FIG. 3 separately will be described.

FIG. 4 shows a configuration of the first embodiment of the apparatusfor multiplexed video data reading according to the present invention,in which a plurality (four in this embodiment) of disk devices DK1, DK2,DK3, and DK4 such as the magnetic disk devices are connected with aplurality of switches SW1, SW2, SW3, and SW4, respectively. Each ofthese switches SW1 to SW4 is also connected with 32 buffer memories BM1to BM32 provided within 32 terminals TE1 to TE32 which also includedecoders DE1 to DE32 and monitors TV1 to TV32, respectively. Theoperations of the disk devices DK1 to DK4, the switches SW1 to SW4, andthe buffer memories BM1 to BM32 are controlled by a control device CN1.

As shown in FIG. 5, in this first embodiment, each compression codedvideo data V1 is divided into segments SG1, SG2, etc., such that eachsegment can be read out in one time-slot at each data storage device,and these divided segments are stored by being sequentially distributedamong the plurality of data storage devices DK1 to DK4 in a mannerindicated in FIG. 5. Namely, the disk device DK1 stores the segmentsSG1, SG5, . . . , SG(4k+1), . . . , the disk device DK2 stores thesegments SG2, SG6, . . . , SG(4k+2), . . . , the disk device DK3 storesthe segments SG3, SG7, . . . , SG(4k+3), . . . , and the disk device DK4stores the segments SG4, SGS, . . . , SG(4k+4), . . . , where k=0, 1, 2,. . . .

In this first embodiment, the multiplexed reading operation iscontrolled according to the timing chart of FIG. 6 as follows.

The control device CN1 generates a plurality (four in this embodiment)of control time-slot sequence CTS1, CTS2, CTS3, and CTS4, each of whichis formed by periodic 32 time-slots TS1, TS2, . . . , TS32 constitutingone period, in correspondence to four sets of the disk device and switchcombination DK1-SW1, DK2-SW2, DK3-SW3, and DK4-SW4. Here, as shown inFIG. 6, each period is divided into four phases with 8 time-slots ineach phase, and four control time-slot sequences have mutually displacedphases such that the control time-slot sequence CTS2 is displaced fromthe control time-slot sequence CTS1 by one phase, the control time-slotsequence CTS3 is displaced from the control time-slot sequence CTS2 byone phase, and the control time-slot sequence CTS4 is displaced from thecontrol time-slot sequence CTS3 by one phase.

Then, the control device CN1 controls each combination of the diskdevice and the switch to carry out the reading of an appropriate videosegment and the transfer of the read out video segment to an appropriatebuffer memory. Here, in the four control time-slot sequences, thetime-slots of the same time-slot number are used for reading the videosegments of the same video data and transferring the read out videosegments to the same buffer memory.

For example, when the first segment SG1 of a certain video data is readout and its transfer target is switched to a certain buffer memory BMk(k=1 to 32) by using the combination DK1-SW1 at the time-slot TS1 of thecontrol time-slot sequence CTS1, the second segment SG2 of the samevideo data is read out and its transfer target is switched to the samebuffer memory BMk by using the combination DK2-SW2 at the same time-slotTS1 of the control time-slot sequence CTS2, the third segment SG3 of thesame video data is read out and its transfer target is switched to thesame buffer memory BMk by using the combination DK3-SW3 at the sametime-slot TS1 of the control time-slot sequence CTS3, the fourth segmentSG4 of the same video data is read out and its transfer target isswitched to the same buffer memory BMk by using the combination DK4-SW4at the same time-slot TS1 of the control time-slot sequence CTS4, and soon.

In this manner, the video segments of each video data are read out andwritten into a certain buffer memory BMk by using the combinations ofthe disk device and the switch sequentially. At the buffer memory BMk,the sequentially entered video segments are sequentially supplied to thedecoder DEk of the same terminal TEk, at which each video segment isdecoded into the video signals. The decoded video signals are thensequentially supplied to the monitor TVk of the same terminal TEk toprovide the display according to the video data.

Here, the video signals contained in one video segment corresponds tothe duration of the phase difference between the successive controltime-slot sequences, so that before the video display of one videosegment part is completed, the reading, the buffer memory writing, andthe decoding of the next video segment are completed, such that the nextvideo signals can be supplied to the monitor before the end of the videodisplay of the previous video segment part, so as to realize thecontinuous video display. In this first embodiment, the period of eachcontrol time-slot sequence has 32 time-slots, so that the independentvideo display can be provided at the monitors of up to 32 terminalssimultaneously.

In FIG. 6, when a video data reading request REQ1 is received from then-th terminal, the idle time-slot is searched out in all of the fourcontrol time-slot sequences CTS1, CTS2, CTS3, and CTS4 starting fromthat moment, and the earliest idle time-slot found in any one of thesefour control time-slot sequences is used to start out the video datareading operation of the requested video data. At this point, as thesuccessive ones of these four control time-slot sequences have the phasedifference of 8 time-slots, regardless of which one of the time-slotsTS1 to TS32 is going to be found as idle, this idle time-slot can befound within a duration of 8 time-slots at most since the arrival of thevideo data reading request REQ1.

For example, when the time-slot TS21 is the only one idle time-slotamong 32 time-slots TS1 to TS32 at a moment, and the video data readingrequest REQ1 arrived at the ending timing of this idle time-slot TS21 onthe control time-slot sequence CTS4 as indicated in FIG. 6. Even in sucha worst possible case, this idle time-slot TS21 can be found on thecontrol time-slot sequence CTS1 which is separated by a duration of 8time-slots from the time-slot TS21 on the control time-slot sequenceCTS4 within the same period, and the video data reading operation forthe requested video data can be started from this time-slot TS21 on thecontrol time-slot sequence CTS1, so that the worst response time in thiscase is the duration of 8 time-slots which is equal to the phasedifference between the successive control time-slot sequences.

Now, when the idle time-slot is selected from the control time-slotsequence CTS2 corresponding to the combination DK2-SW2 for example, thevideo data reading operation is going to start out from the videosegment SG2 stored in the disk device DK2, and the first video segmentSG1 stored in the disk device DK1 is going to be skipped. Similarly,when the idle time-slot is selected from the control time-slot sequenceCTS3 or CTS4, the video segments SG1 and SG2 or SG1, SG2, and SG3 aregoing to be skipped. Thus, depending on the control time-slot sequencefrom which the idle time-slot is to be selected, at most (a number N ofdisk devices-1) video segments are going to be skipped from a topsection of the read out video data.

Here, if one video segment provide 1 second of display, the maximum topskipped part corresponds to 3 seconds of display in this firstembodiment. However, this much of skipped part can be easily concealedin the actual video display by taking an appropriate measure in thevideo data production side, such as a measure which demands therepetition of the title image for the first 4 seconds in each video datafor instance.

Alternatively, it is also possible to compensate this skipping of thevideo segments from a top section of each video data at the terminalside, as will be described as the next second embodiment.

FIG. 7 shows a configuration of the second embodiment of the apparatusfor multiplexed video data reading according to the present invention,in which a plurality (three in this embodiment) of disk devices DK1,DK2, and DK3 such as the magnetic disk devices for storing the videodata V1 are connected with a switching unit SW including a plurality ofswitches in the center 1. Each of these switches of the switching unitSW is also connected with a number of terminals TE50, where eachterminal TE50 includes a data composition unit C1, a data memory unitM1, and a data selection unit TSW1.

In this configuration of FIG. 7, the maximum top skipped partcorresponds to 2 video segments as there are three disk devices DK1 toDk3. Consequently, the data memory unit M1 in the terminal TE50 storesin advance copies of the first two video segments SG1 and SG2 of eachvideo data V1 stored at the center 1 in this case.

Then, when the entire video data is to be displayed, the data selectionunit TSW1 is switched to the data composition unit C1 at which the videosegments of the video data V1 transmitted from the center 1 with at mostfirst two segments skipped are compensated by the video segments SG1 andSG2 stored in the data memory unit M1 to produce the complete video dataV1 without any skipped part.

On the other hand, when the confirmation of each video data V1 alone isto be made, the data selection unit TSW1 is switched to the data memoryunit M1 such that the video segments SG1 and SG2 stored in the datamemory unit M1 are repeatedly displayed. It is also possible to providea video menu by contracting the data of the stored video segments foreach video data and displaying a list of the contracted video displaysfor all the available video data such that the user can select thedesired video data to be requested to the center 1 by watching the topsections of all the available video data on this video menu.

Next, FIG. 8 shows a configuration of the third embodiment of theapparatus for multiplexed video data reading according to the presentinvention, in which a plurality (three in this embodiment) of diskdevices DK1, DK2, and DK3 such as the magnetic disk devices for storingthe video data V1 are connected with a plurality of switches SW1, SW2,and SW3. Each of these switches SW1, SW2, and SW3 is also connected witha number (four in this embodiment) of buffer memories BM1, BM2, BM3, andBM4. Here, the operations of the switches SW1 to SW3 are controlled by atime-slot selection unit SL1 which is connected with a directory memoryunit DR1.

In this configuration of FIG. 8, the directory memory unit DR1 storesthe directory data which indicate whether each video data V1 stores inthe disk devices DK1 to DK3 uses the coding scheme in which the otherframe data referencing range at a time of each frame decoding is alwaysclosed within a limited range and that limited range is always withinone segment, as in a case of the MPEG (Motion Picture Experts Group)coding of the ISO-11172 with the segment length set to be a positiveinteger multiple of GOP (Group Of Pictures) in the MPEG, or the codingscheme in which the other frame data referencing range at a time of eachframe decoding is not necessarily closed within a limited range and thereferencing range can possibly be stretched over more than one segments,as in a case of the H. 261 coding of the ITU-T.

Then, using this directory data in the directory memory unit DR1, thetime-slot selection unit SL1 operates at a time of reading out eachvideo data V1 from the disk devices DK1 to DK3 according to the flowchart of FIG. 9, as follows. When the video data reading request for thevideo data V1 is received at the time-slot selection unit SL1 (step S1),the directory data stored in the directory memory unit DR1 are searchedthrough (step S2) to judge whether the coding scheme used for the videodata V1 to be read out is a coding scheme in which the other frame datareferencing range at a time of each frame decoding is always closedwithin one segment or a coding scheme in which the other frame datareferencing range at a time of each frame decoding is not necessarilyclosed within one segment (step S3).

In a case of the coding scheme in which the other frame data referencingrange at a time of each frame decoding is always closed within onesegment, just as in the first embodiment described above and asindicated in FIG. 10, the utilization state of te time-slots in thecontrol time-slot sequences CTS1 to CTS3 corresponding to the switchesSW1 to SW3 is checked during a period of time TM1 (corresponding to onephase of the period of the control time-slot sequences) covering as manytime-slots as a quotient (equal to two in this embodiment) of a totalnumber (six in this embodiment) of the periodic time-slots divided by atotal number of disk devices (three in this embodiment), starting from atiming P1 at which the judgement of the coding scheme of the video dataV1 to be read out is completed, and the earliest idle time-slot isselected from any of the control time-slot sequences CTS1 to CTS3 (stepS4).

When the time-slot TS3 on the control time-slot sequence CTS3 isselected for example, the video segments SG3, SG4, etc. of this videodata V1 are sequentially read out from the disk devices DK1 to DK3,starting from the disk device DK3 corresponding to this controltime-slot sequence CT3, by using the selected time-slot TS3 in each oneof the control time-slot sequences CTS1 to CTS3 which appears in ancyclic order as indicated in FIG. 10. Then, the video data V1 isreconstructed at the transfer target buffer memory such as BM1 forexample by composing all the video segments SG3, SG4, etc. transmittedfrom the disk devices DK1 to DK3 through the switches SW1 to SW3. Inthis case, the first two video segments SG1 and SG2 are going to beskipped, but since each video segment usually corresponds to about 1second of video display so that the total 2 seconds of the skipped videosegments at the top section of the video data can be regarded as not asmuch as seriously affecting the quality of the read out video data.

On the other hand, in a case of the coding scheme in which the otherframe data referencing range at a time of each frame decoding is notnecessarily closed within one segment, the utilization state of tetime-slots in the control time-slot sequence CTS1 corresponding to thedisk device DK1 which stores the first video segment SG1 is checkedduring a period of time TM2 (corresponding to one period of the controltime-slot sequences) covering as many time-slots as a total number (sixin this embodiment) of the periodic time-slots, starting from a timingP1 at which the judgement of the coding scheme of the video data V1 tobe read out is completed, and the earliest idle time-slot is selectedonly from this control time-slot sequence CTS1 (step S5). Thus, in thiscase, the reading of the video segments of the video data always startsfrom the disk device DK1 corresponding to this control time-slotsequence CTS1 which stores the first video segment SG1, and consequentlyall the video segments of the requested video data can be read outwithout skipping any video segment.

When the time-slot TS3 on the control time-slot sequence CTS1 isselected for example, the video segments SG1, SG2, etc. of this videodata V1 are sequentially read out from the disk devices DK1 to DK3,starting from the disk device DK1 corresponding to this controltime-slot sequence CT1, by using the selected time-slot TS3 in each oneof the control time-slot sequences CTS1 to CTS3 which appears in ancyclic order as indicated in FIG. 10. Then, the complete video data V1is reconstructed at the transfer target buffer memory such as BM1 forexample by composing all the video segments SG1, SG2, etc. transmittedfrom the disk devices DK1 to DK3 through the switches SW1 to SW3.

In this case of the coding scheme in which the other frame datareferencing range at a time of each frame decoding is not necessarilyclosed within one segment, the response time is going to be at longestthe period of time TM2 which may be longer than the response time up tothe period of time TM1 in the above described case of the coding schemein which the other frame data referencing range at a time of each framedecoding is always closed within one segment, bat there is going to beno skipping of the video segment in the read out video data, so that itbecomes possible to prevent the serious damage on the video display forthe top section of the video data due to the lack of the appropriatereferencing frame data at a time of the video data decoding.

As described, according to the present invention, it is possible tosecure the idle time-slot within each phase of each period which isequal to the phase difference between successive control time-slotsequences in each period, so that the worst response time can beshortened to be equal to the duration of each phase of the period of thetime-slot sequences. For example, in the above described case of thefirst embodiment of FIGS. 4 to 6, the worst response time is as much asthe duration of 8 time-slots, so that when one time-slot corresponds to125 ms, the worst response time can be reduced from 4 seconds in theconventional case to 1 second in the first embodiment of the presentinvention, i.e., the worst response time can be shortened to 1/4compared with the conventional case.

In addition, the present invention does not require the shortening ofeach time-slot duration, so that the same average reading speed from thedata storage device as in the conventional video library system can bemaintained and the same level of multiplexing as in the conventionalvideo library system can be provided while reducing the worst responsetime.

Moreover, according to the above described second embodiment of thepresent invention, it becomes possible to obtain the complete video dataat the terminal side by compensating any skipped video segments at a topsection of the video data by the partial video data provided on theterminal side in advance, while it also becomes possible to provide avideo display of a part of each video data on the terminal side by usingthe same partial video data, so that the confirming of each video datacan be made at the terminal side without requiring the access to thecenter for this purpose alone.

Furthermore, according to the above described third embodiment of thepresent invention, it becomes possible to dynamically change the idletime-slot selection rule According to the coding scheme of the videodata to be read out, so that the fast response time can be secured for acase of the coding scheme in which the other frame data referencingrange at a time of each frame decoding is always closed within onesegment, while the serious damage on the video display for the topsection of the video data can be prevented for a case of the codingscheme in which the other frame data referencing range at a time of eachframe decoding is not necessarily closed within one segment. This thirdembodiment is particularly effective in a case the video datacompression coded by more than one coding schemes such as the MPEGcoding scheme and the H. 261 coding scheme are mixedly present.

It is to be noted that, besides those already mentioned above, manymodifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

What is claimed is:
 1. An apparatus for multiplexed video data reading,comprising:N data storage means for distributedly storing a plurality ofvideo data divided into segments, where each segment is capable of beingread out in one time-slot and N is an integer greater than one; N switchmeans, provided in correspondence to said N data storage means, forselectively connecting each data storage means with one of a pluralityof data transfer targets; and a control device for generating N controltime-slot sequences formed by periodic time-slots in correspondence tosaid N data storage means with mutually displaced phases in each periodof said N control time-slot sequences, and controlling said N datastorage means and said N switch means such that, in response to arequest for reading one video data from one data transfer target, anearliest available idle time-slot is selected from said N controltime-slot sequences, and the segments of said one video data aresequentially read out from said N data storage means at time-slotscorresponding to said earliest idle time-slot in said N controltime-slot sequences and transmitted to said one data transfer target assaid N data storage means are sequentially switched to said one datatransfer target by said N switch means.
 2. The apparatus of claim 1,wherein each period of said N control time-slot sequences is dividedinto N phases, such that successive control time-slot sequences have aphase difference equal to one phase.
 3. The apparatus of claim 1,wherein the segments of said one video data are sequentially read outstarting from one data storage means corresponding to one controltime-slot sequence in which said earliest available idle time-slot isselected.
 4. The apparatus of claim 1, wherein the segments of the videodata are distributedly stored in sequential order among said N datastorage means.
 5. The apparatus of claim 1, wherein the data transfertargets are buffer memories provided in terminals of a video librarysystem while said N data storage means, said N switch means and saidcontrol device are provided in a center of the video library system. 6.The apparatus of claim 1, further comprising data memory means, providedin correspondence to each data transfer target, for storing in advance apartial video data for each video data stored in said N data storagemeans including a prescribed number of segments from a top section ofeach video data, and data composition means, provided in correspondenceto each data transfer target, for composing a complete video data foreach video data from the segments of each video data read out andtransmitted from said N data storage means and the partial video datafor each video data stored in the data memory means.
 7. The apparatus ofclaim 6, wherein the data memory means stores at least (N-1) segmentsfrom a top section of each video data.
 8. The apparatus of claim 6,further comprising data selection means, provided in correspondence toeach data transfer target, for selectively outputting one of the partialvideo data stored by the data memory means and the complete video datacomposed by the data composition means.
 9. The apparatus of claim 1,wherein the control device includes:means for distinguishing a codingscheme used for each video data as either a coding scheme in which otherframe data referencing range at a time of each frame decoding is alwaysclosed within one segment or a coding scheme in which other frame datareferencing range at a time of each frame decoding is not necessarilyclosed within one segment; and means for changing an idle time-slotselection rule used by the control device according to the coding schemedistinguished by the distinguishing means, such that the earliestavailable idle time-slot is selected among all of said N controltime-slot sequences in a case of the coding scheme in which other framedata referencing range at a time of each frame decoding is always closedwithin one segment, whereas the earliest available idle time-slot isselected from one control time-slot sequence corresponding to one datastorage means which stores a first segment of each video data in a caseof the coding scheme in which other frame data referencing range at atime of each frame decoding is not necessarily closed within onesegment.
 10. The apparatus of claim 9, wherein the earliest availableidle time-slot is selected within as many time-slots of each of said Ncontrol time-slot sequences as a total number of periodic time-slots ineach period of said N control time-slot sequences divided by N in a caseof the coding scheme in which other frame data referencing range at atime of each frame decoding is always closed within one segment, whereasthe earliest available idle time-slot is selected within as manytime-slots in said one control time-slot sequence as a total number ofperiodic time-slots in each period of said one control time-slotsequence in a case of the coding scheme in which other frame datareferencing range at a time of each frame decoding is not necessarilyclosed within one segment.
 11. A method of multiplexed video datareading, comprising the steps of:distributedly storing a plurality ofvideo data divided into segments among N data storage means, where eachsegment is capable of being read out in one time-slot and N is aninteger greater than one; selectively connecting each data storage meanswith one of a plurality of data transfer targets by N switch meansprovided in correspondence to said N data storage means; generating Ncontrol time-slot sequences formed by periodic time-slots incorrespondence to said N data storage means with mutually displacedphases in each period of said N control time-slot sequences; andcontrolling said N data storage means and said N switch means such that,in response to a request for reading one video data from one datatransfer target, an earliest available idle time-slot is selected fromsaid N control time-slot sequences, and the segments of said one videodata are sequentially read out from said N data storage means attime-slots corresponding to said earliest idle time-slot in said Ncontrol time-slot sequences and transmitted to said one data transfertarget as said N data storage means are sequentially switched to saidone data transfer target by said N switch means.
 12. The method of claim11, wherein at the generating step, each period of said N controltime-slot sequences is divided into N phases, such that successivecontrol time-slot sequences have a phase difference equal to one phase.13. The method of claim 11, wherein at the controlling step, thesegments of said one video data are sequentially read out starting fromone data storage means corresponding to one control time-slot sequencein which said earliest available idle time-slot is selected.
 14. Themethod of claim 11, wherein at the storing step, the segments of thevideo data are distributedly stored in sequential order among said Ndata storage means.
 15. The method of claim 11, wherein at theselectively connecting step, the data transfer targets are buffermemories provided in terminals of a video library system while said Ndata storage means and said N switch means are provided in a center ofthe video library system.
 16. The method of claim 11, further comprisingthe steps of:additionally storing in advance a partial video data foreach video data stored in said N data storage means including aprescribed number of segments from a top section of each video data bydata memory means provided in correspondence to each data transfertarget; and composing a complete video data for each video data from thesegments of each video data read out and transmitted from said N datastorage means and the partial video data for each video data stored inthe data memory means by data composition means provided incorrespondence to each data transfer target.
 17. The method of claim 16,wherein at the additionally storing step, the data memory means storesat least (N-1) segments from a top section of each video data.
 18. Themethod of claim 16, further comprising the step of selectivelyoutputting one of the partial video data stored by the data memory meansand the complete video data composed by the data composition means. 19.The method of claim 11, further comprising the steps of:distinguishing acoding scheme used for each video data as either a coding scheme inwhich other frame data referencing range at a time of each framedecoding is always closed within one segment or a coding scheme in whichother frame data referencing range at a time of each frame decoding isnot necessarily closed within one segment; and changing an idletime-slot selection rule used at the controlling step according to thecoding scheme distinguished at the distinguishing step, such that theearliest available idle time-slot is selected among all of said Ncontrol time-slot sequences in a case of the coding scheme in whichother frame data referencing range at a time of each frame decoding isalways closed within one segment, whereas the earliest available idletime-slot is selected from one control time-slot sequence correspondingto one data storage means which stores a first segment of each videodata in a case of the coding scheme in which other frame datareferencing range at a time of each frame decoding is not necessarilyclosed within one segment.
 20. The method of claim 19, wherein at thecontrolling step, the earliest available idle time-slot is selectedwithin as many time-slots of each of said N control time-slot sequencesas a total number of periodic time-slots in each period of said Ncontrol time-slot sequences divided by N in a case of the coding schemein which other frame data referencing range at a time of each framedecoding is always closed within one segment, whereas the earliestavailable idle time-slot is selected within as many time-slots in saidone control time-slot sequence as a total number of periodic time-slotsin each period of said one control time-slot sequence in a case of thecoding scheme in which other frame data referencing range at a time ofeach frame decoding is not necessarily closed within one segment.